Multiple voltage integrated circuit and design method therefor

ABSTRACT

An integrated circuit (IC) design, method and program product for reducing IC design power consumption. The IC is organized in circuit rows. Circuit rows may include a low voltage island powered by a low voltage (V ddl ) supply and a high voltage island powered by a high voltage (V ddh ) supply. Circuit elements including cells, latches and macros are placed with high or low voltage islands to minimize IC power while maintaining overall performance. Level converters may be placed with high voltage circuit elements.

CROSS REFERENCE TO RELATED APPLICATION

The present application is a divisional application of U.S. Pat. No.7,111,266 application Ser. No. 10/720,464, filed Nov. 24, 2003 entitled“MULTIPLE VOLTAGE INTEGRATED CIRCUIT AND DESIGN METHOD THEREFOR” toAnthony Correale, Jr. et al.; and related to U.S. Pat. No. 7,089,510application Ser. No. 10/720,562 entitled “METHOD AND PROGRAM PRODUCT OFLEVEL CONVERTER OPTIMIZATION” to Anthony Correale Jr. et al., U.S. Pat.No. 7,119,578 application Ser. No. 10/720,466 entitled “SINGLE SUPPLYLEVEL CONVERTER” to Anthony Correale Jr. et al., both filed coincidentwith the parent application and to U.S. Pat. No. 7,091,574 applicationSer. No. 10/387,728 entitled “VOLTAGE ISLAND CIRCUIT PLACEMENT” toAnthony Correale Jr., all assigned to the assignee of the presentinvention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is related to integrated circuit (IC) designcircuit design and more particularly, to optimizing standard cell designconfigurations.

2. Background Description

Semiconductor technology and chip manufacturing advances have resultedin a steady increase of on-chip clock frequencies, the number oftransistors on a single chip and the die size itself, coupled with acorresponding decrease in chip supply voltage and chip feature size.Generally, all other factors being constant, the power consumed by agiven clocked unit increases linearly with the frequency of switchingwithin it. Thus, not withstanding the decrease of chip supply voltage,chip power consumption has increased as well. Both at the chip andsystem levels, cooling and packaging costs have escalated as a naturalresult of this increase in chip power. For low end systems (e.g.,handhelds, portable and mobile systems), where battery life is crucial,net power consumption reduction is important but, must be achievedwithout degrading performance below acceptable levels. Consequently,power consumption has been a major design consideration for designingvery large scale integrated circuits (VLSI) such as high performancemicroprocessors. In particular, increasing power requirements runcounter to the low end design goal of longer battery life. Since chippower is directly proportion to the square of supply voltage (V_(dd)),reducing supply voltage is one of the most effective ways to reduce thepower consumption, both active and standby (leakage) power, which isbecoming more and more of a problem as technology features scale intonanometer (nm) dimension range.

While reducing supply voltage is attractive to reduce the powerconsumption, reducing V_(dd) increases transistor and gate delay. Thus,for a design that is performance constrained, the supply voltage may notbe lowered too much and, it is usually determined by the most timingcritical paths. However, it is often the case that most cells in a chipare timing non-critical. If those timing non-critical cells are properlyselected to be on lower supply voltage(s), significant power saving maybe achieved without degrading the overall circuit performance.

One approach to reducing power is to use multiple supply voltages eachsupplying different circuit blocks or voltage islands. Each voltageisland runs at its minimum necessary supply voltage. However, multiplesupply voltages on the same circuit/chip present numerous problems,especially for deep submicron (DSM) designs, where circuit performanceoften is dominated by interconnect delays. In particular, logicsynthesis is very complicated for multiple supply designs and, placementand routing must be considered together for voltage assignment, levelconverter insertion and minimization, and for circuit block clusteringto simplify power routing of multiple supply lines.

Thus, there is a need for circuit element clustering for minimum powerand to simplify power routing of multiple supply lines.

SUMMARY OF THE INVENTION

It is a purpose of the invention to improve integrated circuit (IC) chipdesign;

It is another purpose of the invention to improve cell placement inmulti supply voltage IC chip designs;

It is yet another purpose of the invention to improve cell placement offirst supply voltage cells with cells of other supply voltages in multisupply voltage IC chip designs;

It is yet another purpose of the invention to group circuit cells in amulti-supply design close to their respective power supplies;

It is yet another purpose of the invention to group circuit cells in amulti-supply design to facilitate timing closure;

It is yet another purpose of the invention to group circuit cells in amulti-supply design for optimum level converter placement;

It is yet another purpose of the invention to group circuit cells in amulti-supply design for a minimum number of level converters;

It is yet another purpose of the invention to group circuit cells in amulti-supply design for efficient level converter placement.

The present invention relates to an integrated circuit (IC) design,method and program product for reducing IC design power consumption. TheIC is organized in circuit rows. Circuit rows may include a low voltageisland powered by a low voltage (V_(ddl)) supply and a high voltageisland powered by a high voltage (V_(ddh)) supply. Circuit elementsincluding cells, latches and macros are placed with high or low voltageislands to minimize IC power while maintaining overall performance.Level converters may be placed with high voltage circuit elements.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, aspects and advantages will be betterunderstood from the following detailed description of a preferredembodiment of the invention with reference to the drawings, in which:

FIGS. 1A-C show different state of the art circuit layouts formulti-supply chips;

FIG. 2 shows an example of a generic voltage island structure formedaccording to a preferred embodiment of the present invention;

FIG. 3 shows an example of a flowchart of a method of generic voltageisland optimization for low power with rapid timing closure according toa preferred embodiment of the present invention;

FIGS. 4A-B show an example of the steps in the logic aware voltageassignment;

FIGS. 5A-B show an isolated V_(ddl) cell (e.g., width 1 cell) in themiddle of a larger V_(ddh) island, optimized by changing such isolatedcells back to a V_(ddh) cell;

FIGS. 6A-F show before and after level converter placement examples,optimized according to a preferred embodiment of the present invention;

FIGS. 7A-B show an example of a V_(ddl) fanin cone for an iterativelevel converter optimization;

FIG. 8 shows an example of level converter efficiency measurement flowdiagram using V_(ddl) fanin cone size to iteratively locate and deleteleast efficient level converters;

FIGS. 9A-B show before and after examples of level converteroptimization effected with logic replacement;

FIG. 10 shows a flow diagram showing an example of the logicreplacement;

FIGS. 11A-B show before and after examples of replacing a buffer andlevel converter with a single level converter and adjusting placement tomeet design objectives;

FIG. 12 shows a flow diagram for identifying paired level converters andbuffers for optimization.

DESCRIPTION OF PREFERRED EMBODIMENTS

Accordingly, as described hereinbelow, the present invention provides aversatile and generic multi-supply voltage island circuit structure,wherein different supply voltages are assigned at both macro and celllevel within the islands. Unless indicated otherwise for simplicity ofdiscussion hereinbelow, logic cell and gate are used interchangeably andeach is a sub-circuit of standard cell design. Further, a standard celldesign is taken as having the same height, i.e., row height, for mostcells. Abutting cells form circuit rows. Also, typical modem applicationspecific integrated circuit (ASIC) and system on a chip (SOC) designsoften have many proprietary macros (known in the art as intellectualproperty (IP) blocks) mixed with standard cells. A voltage island can bea single cell, an IP block or macro or, a continuous region of cells onthe same or adjacent rows that have the same power supply voltage(referred to herein as a high voltage supply or V_(ddh) and a lowvoltage supply or V_(ddl)). An output or source drives a net connectingone or more inputs or sinks to the source and a low/high voltage netconnects a low/high voltage source to low/high voltage sinks. Also,although described herein in terms of two (2) supplies description, thisis for example only and not intended as a limitation. A person skilledin the art would readily understand how to extended the 2 supplydescription to multiple supply voltages.

So, FIGS. 1A-C show different state of the art multi-supply chips withexamples of well known circuit island placement, e.g., as described inD. E. Lackey et al., “Managing power and performance for system-on-chipdesigns using voltage islands”, in Proc. International Conference onComputer Aided Design, pp. 195-202, November 2002. In the example 100 ofFIG. 1A, voltage islands are only allowed at the macro level 102, 104,with no fine-grained voltage assignment for cells 106, 108. For deepsubmicron (DSM) designs, which have tight performance targets, it maynot be possible to switch an entire macro between a normal and a lowersupply voltage without incurring an overall circuit performance loss. Soit would be more flexible if voltage assignment can be done at celllevel to exploit positive slacks. The example of FIG. 1B shows a circuitblock 110 with cell level voltage assignment, but at the cost of arestricting the layout to alternating or interleaving pairs of high andlow supply rows 112, 114. FIG. 1C shows another example 120, somewhatunconstrained by the requirement of interleaving entire rows. Instead,in this example each row 122, 124, 126, 128 may have two areas withdifferent voltages (designated H or L), provided each area occupieseither the left or right part of the row. Unfortunately, these voltageisland patterns or segregated voltage areas unnaturally constrainvoltage assignment and/or reduce placement flexibility. Frequently in atypical modern ASIC/SOC design, non-critical regions are interspersedwith critical regions in the same circuit row. Typically available suchcircuit structures are not flexible enough to allow circuit placement orvoltage island granularity sufficient to meet stringent delayconstraints or, in placing to meet such constraints introduce routingproblems.

By contrast, a preferred circuit and chip design method incorporatesgeneric voltage islands with much finer layout granularity. Supplyvoltage assignment may be done at both macro and gate level, affordingdesigners much more design freedom and providing a much more flexiblevoltage island layout structure. Further such a preferred embodimentdesign achieves timing closure on design timing goals during voltageisland formation and hastens timing optimization.

FIG. 2 shows an example of a generic voltage island structure 130 formedaccording to a preferred embodiment of the present invention, whereindifferent voltages are assigned at both macro and cell levels. Preferredvoltage assignment affords more freedom in terms of layout style byallowing multiple voltage islands within the same circuit row. Further,such a pattern 130 is achievable with minimum disturbance to an existingplacement, i.e., after normal chip design and placement. So, afterdesigning and placing circuits for performance, for example, the designmay be modified according to the present invention, selectivelyreplacing higher power (V_(ddh)) circuits (stippled) with lower power(V_(ddl)) circuits (clear) where possible. Since some gap may be neededbetween adjacent V_(ddl) islands 132 and V_(ddh) islands 134 (dependingon the standard cell library), a minimum or maximum allowed cluster sizeor number of voltage islands may be specified for each circuit row,e.g., 136, based on the particular user or technology specification.See, for example, U.S. application Ser. No. 10/387,728 entitled “VOLTAGEISLAND CIRCUIT PLACEMENT” to Anthony Correale Jr., filed Mar. 13, 2003,assigned to the assignee of the present invention and incorporatedherein by reference. To facilitate power routing, a power grid structureof VDDL 138 and VDDH 140 is co-designed with the voltage islandassignment.

Typically, a V_(ddl) source cannot drive a V_(ddh), sink reliablywithout excessive leakage. Thus, a level converter is needed for atransition from a low voltage net to a high voltage net. Traditionallevel converters require both supply voltages, V_(ddl) and V_(ddh), toavoid excessive leakage. Previously, using dual-supply voltage levelconverters required that they be placed at the island 132, 134boundaries for access to both power supplies. However, a single-supplylevel converter is used such as is described in U.S. Pat. No. 7,119,578entitled “SINGLE SUPPLY LEVEL CONVERTER” to Anthony Correale Jr. et al.,filed coincident with the parent to this application and incorporatedherein by reference. Correale Jr. et al. level converters 144 can beplaced anywhere in a higher voltage island 134 or logic 146 and so,provide additional placement flexibility. Preferably, a level convertersas described hereinbelow is a single supply level converter such asCorreale Jr. et al.

FIG. 3 shows an example of a flowchart 150 of a method of genericvoltage island optimization for low power with rapid timing closureaccording to a preferred embodiment of the present invention. For deepsubmicron (DSM) designs, interconnect delay can dominate the transistordelay, thus placement (and even routing) information are used to get anaccurate timing estimation.

So, beginning in step 152 an input netlist description andspecifications (e.g., technology files and timing constraints) isprovided. In step 154 a timing closure tool with Spice RC delays (e.g.,a suitable tool from Synopsis, Inc., or EinsTimer from IBM Corporation)is used to determine the entire circuit/chip timing at the higher supplyvoltage (V_(ddh)) for a base placement and optimization, i.e.,determining global placement and obtaining a good timing estimation.Then, non-critical cells are identified and assigned a lower supplyvoltage (V_(ddl)). As noted hereinabove, interconnect delay can dominatethe gate delay for deep submicron circuits and so, power can be reducedfor lightly loaded circuits where power is not needed for driving largeinterconnect loads. So, the global placement information is used tocorrectly identify the critical versus non-critical cells, e.g., heavilyloaded verses lightly loaded. Then in step 156, a logic aware voltageassignment is performed, assigning the lower supply voltage(s) to lesscritical circuits, i.e., macro, latch and/or cell. Next, in step 158level converters are inserted and the results are refined and optimized.A level converter is inserted wherever there is a transition net with alow voltage cell driving a high voltage cell or, where a pass gate datainput to a low voltage cell or circuit element is being driven by a highvoltage cell and being controlled by a low voltage cell. In step 160isolated assignments are removed in a physical aware voltagereassignment step, locating and reverting solo or very small groups oflow voltage circuits that are difficult to form into low voltageislands. Since eliminating those isolated low voltage cells may createopportunities to reassign previously assigned high voltage cells to lowvoltage cells, in step 162 the design is checked for such opportunities.If any are found, returning to step 156 for another pass the design isfurther optimized, until there is no improvement available in step 162.Finally, in step 164 placement and power routing patterns are effectedbased on the voltage island assignments to form the final high and lowvoltage islands. As result, the entire flow can be tightly integratedwith a suitable physical synthesis engine 166 such as a routing toolfrom Cadence Design Systems, e.g., for application of any necessaryfurther timing optimization.

In addition to identifying circuits for separation into voltage islands,supply high and low voltages may similarly be selected to achieveoptimum power saving. Further, a preferred voltage assignment method hasapplication to static and incremental timing engines. Every time a macroor cell is changed from a higher voltage cell to a lower voltage cell,or vice verse, the timing (slack) is updated.

FIGS. 4A-B show an example of the steps in the logic aware voltageassignment step 156 of FIG. 3. Essentially, a logic assessment is donefor each macro 1560, latch 1562 and cell 1564 to determine which may bereplaced with a low voltage equivalent and level converter, if required.For checking combinational logic cells in step 1564, the cells may besorted according to timing order from timing end point to timingstarting point, i.e., from PO to PI or latch input to latch output. Ineach major step 1560, 1562 and 1564, each circuit element of each group(macro, latch or cell) is checked, essentially according to the steps1570-1576 in FIG. 4B to identify low voltage candidates. First in step1570, the supply to the macro, latch or cell is reduced and one or morelevel converters are inserted where appropriate, i.e., at transitionnets with low voltage sources driving high voltage sinks. In step 1571an appropriate incremental timing report is checked for the macro, latchor cell. Then, in step 1572, if the timing specification of the macro,latch or cell is met, it is designated to the low supply voltage. Forlatches in particular, a latch is designated a low supply latch, if allinput pins still have positive slack (i.e., edges arrive at inputs priorto a minimum input set up time) and the output pin slack exceeds aminimum threshold, i.e., for a transitional net the output canaccommodate the additional delay for an inserted level converter.Otherwise, in step 1573 it is reverted to the normal, higher supply. Instep 1574, if additional macros, latches or cells have not yet beenchecked; then in step 1575, the next (macro, latch or cell) is selectedand returning to step 1570, checking continues. Once, each element ofthe particular group being checked, i.e., in step 1560, 1562 or 1564,checking proceeds to the next group in 1562 or 1564, respectively, orends in step 1576. After an initial voltage assignment, the voltageassignment may be further refined, including deleting smaller lowvoltage supply clusters.

The initial voltage assignment is not physically aware, i.e., noconsideration is given to cell placement. As shown in the example ofFIG. 5A, it is possible to assign an isolated V_(ddl) cell 170 (e.g.,width 1 cell) in the middle of a larger V_(ddh) island, 172A-B, 174,176, 178. Since such an isolated placement may make it difficult to formuniform voltage islands, an optimum placement is facilitated by changingeach such isolated cell 170 back to a V_(ddh) cell 170′ as shown in FIG.5B. It should be noted that initial assignment of these isolated V_(ddl)cells may have prohibited considering other V_(ddl) cells as candidates.Thus, a physical aware voltage reassignment is employed to push morecells to V_(ddl) while minimizing the number of level converters andstill meeting the physical timing constraints. So, physical adjacencyinformation is used to facilitate the physical aware voltagereassignment and to guide subsequent voltage assignment.

Physical aware voltage reassignment step 160 in FIG. 3, basically,includes 2 steps. First, a physical adjacency metric (PAM) is computedfor the each V_(ddl) cell. The PAM(k, d) for each particular V_(ddl)cell is, the total size (i.e., width) of V_(ddl) cells within theneighboring k rows, including the cell itself, and within diameter ranged. Then, all V_(ddl) cells with a PAM less than certain threshold arereverted to V_(ddh) cells. Each reversion may present new opportunitiesfor converting some other V_(ddh) cells that had not been selected inthe initial voltage assignment, e.g., due to slack constraints. So, instep 162 of FIG. 3 logic aware voltage assignment is called again withPAM as an additional metric. Only those cells with PAM larger or equalto the selected threshold may be selected as V_(ddl) cells. Thus, thelogic aware assignment step 156 and physical aware reassignment step 162may be iterated until no further improvement is realized.

In each iteration level converter placement is optimized in step 158 toreduce the total number of level converters, gradually deleting the lessefficient level converters. Level converters are necessary fortransitions between islands, i.e., at least when a V_(ddl) source isdriving a V_(ddh) sink. So, for example, branches to those levelconverters with a small V_(ddl) fanin may be eliminated (deleting thelevel converter and returning the prior cell with a V_(ddl) cell) oranother level converter efficiency metric may be used to select levelconverters for deletion. Further, since level converters and buffersessentially have the same function and so, can be substituted forbuffers, optimizing level converters, simultaneously optimizes buffers.In particular, for any V_(ddl) output driving multiple V_(ddh) inputs(i.e., inputs to multiple V_(ddh) cells), instead of inserting a levelconverter for each V_(ddh) input, a single level converter is shared,provided that timing and electrical constraints are still met.

FIGS. 6A-F show before and after level converter placement examples. Inthe example of FIG. 6A, a V_(ddl) driver 180 is shown driving atransition net with two V_(ddh) receivers 182, 184 aligned in a straightline, where the level converter 186 is at the geometric center of thetwo receivers 182, 184. However, this placement increases the total wirelength because of the detour from the driver 180 to the level converter186 and, then to the left receiver 182. By contrast, as shown in FIG.6B, an optimized placement places the level converter 186 just in frontof the left receiver 182 to minimize the total net power by maximizingthe low voltage net length portion. Thus, power saving may notnecessarily decrease the total wire length, but optimizes itsapportionment.

Similarly, as shown in the examples of FIGS. 6C-D, placement can beoptimized for a driver 190 driving a transition net with receivers 192,194, 196, 198 on a two dimensional plane from the driver 190. In thisexample, the receivers 192, 194, 196, 198 are all located in the firstquadrant from the perspective of the driver 190. A common levelconverter 200 can be shared between V_(ddl) and V_(ddh) interfaces.Preferably, however, the optimum level converter 200 placement is alocation to minimize the total wire length; and also, allocates thelargest portion of that wire length to the low supply voltage side(i.e., driven by the V_(ddl) driver 190) to minimize switching power,i.e., power expended driving the wire load. Thus, in the example of FIG.6C the level converter 200 is located a minimum power point at (X_(min),Y_(min)), where X_(min) and Y_(min) are the minimum x and y coordinatesof all receivers 192, 194, 196, 198. Thus selecting the minimum powerpoint avoids any total wire length increase, but may place the levelconverter 200 closer to the driver 190. Alternatively, in FIG. 6D thelevel converter 200 may be placed at the Manhattan distance from thenearest sink (receiver 194 in this example) to the source (on the 45°dotted line 202 in this example). A weighted geometric center 204 may bedetermined for all the receivers 192, 194, 196, 198 from a delay neutraldrive point from the level converter 200. The weight applied for eachreceiver 192, 194, 196, 198 is a measure of how close the receivershould be to the driver 190 (e.g., the weight may be measured by theslack at each receiver). Then, a projection is determined from theweighted geometric center 204 to the 45° dotted line 202 is performed todetermine the level converter location. The weighted center placementmore aggressively pushes the level converter 200 further away from thesource 190 to increase the total V_(ddl) wire length and thus reduceV_(ddh) wire length, and as a result, minimize power.

FIGS. 6E-F show after placement examples, wherein V_(ddh) receivers 210,212, 214, 216 are located in more than just a single quadrant, e.g.,they occupy both the first and the fourth quadrant. In this example, thelevel converter 218 is placed at a side drive point (X_(min), Y_(drv)),where X_(min) is the minimum x-coordinate of all receivers, and Y_(drv)is the y-coordinate of the driver 220. Similar drive points can belocated for first-second quadrants, second-third quadrants, andthird-fourth quadrants. However, if as in the example of FIG. 6F, thereceivers 230, 232, 234, 236 238 are dispersed in diagonal quadrants(e.g., first-third quadrants, or second-fourth quadrants), the levelconverter 240 is placed near the driver 242 because it may not beinserted at any other place without increasing the total wire length.

It should be noted that in all of the above examples, if one levelconverter 186, 200, 218, 240 is not enough to drive all the respectiveV_(ddl) receivers, it may be powered up using any suitable technique,e.g., cloning. Whether the level converter is powered up through cloningor otherwise should be evaluated together with the overall power savingof the placement. In particular, the original assignment of V_(ddl)driver may be reverted to V_(ddh) if the level converter cost is higherthan the gain by selecting the driver to be V_(ddl) in the first place.Furthermore, level converter placement as described with reference toFIGS. 6A-F is done focusing on total power saving, by minimizing theoverall capacitance and V_(ddh) cell load capacitance, while maximizingthe V_(ddl) cell load capacitance after level converter placement.However, application of the above described level placement may be doneguided by any other selected cost function, such as timing and powersupply adjacency, i.e., to deliver proper power supplies to levelconverters. After the level converter is inserted and placed, a Steinertree is constructed to connect the level converter with the V_(ddh)receivers.

FIGS. 7A-B show an example of an iterative optimization of levelconverter placement for a V_(ddl) fanin cone 250 according to apreferred embodiment of the present invention. Generally, a fanin conefor level converter includes all gates that drive nets leading to thegate inputs and, as applied to the level converters, signals originatingfrom V_(ddl) gates without crossing/passing through any V_(ddh) gates.As a rule of thumb, the larger the V_(ddl) fanin cone, the moreeffective the level converter.

In this example the V_(ddl) fanin cone 250 for level converter 252includes the 5 gates 254, 256, 258, 260, 262. In this example, the sizeof each V_(ddl) fanin cone for the level converters 252, 266 and 268 is5, 1 and 4, respectively. However, since each level converter 252, 266,268 consumes power and chip area, placement is optimized by deletinginefficient level converters. To the first order, the size of V_(ddl)fanin cone is a rough measure of the efficiency of a particular levelconverter. So, level converters that are inefficient, i.e., levelconverters with small fanin cones, are deleted. For example, the levelconverter 266, which has V_(ddl) fanin cone size of one (i.e., only onebuffer 270 driving into it) and so, may not be cost effective withrespect to power or area. Further, as shown in FIG. 7B after deletinglevel converter 266 and reverting the single, low voltage input buffer270 to V_(ddh) buffer 272, the inefficient fanin cone has beeneliminated. Also, after deleting level converter 266, the V_(ddl) fanincones of level converters 252 and 268 are 4 and 4, respectively.

FIG. 8 shows an example of level converter efficiency measurement flowdiagram 280 using V_(ddl) fanin cone size to iteratively locate anddelete least efficient level converters according to a preferredembodiment of the present invention. First, in step 282 the V_(ddl)fanin cone of each level converter is determined. Then, in step 284level converters with a fanin having a cone size less than or equal to aselected threshold, k, are converted to V_(ddl) cells. Next in step 286the V_(ddl) fanin cone size for remaining level converters is updated.In step 288 fanin cones are checked to determine whether moreinefficiently placed level converters can be removed, i.e., have a fanincone size below k. If more fanin cones with a size below k remain, then,returning to step 284, remaining such inefficient level converters areremoved, one at a time until none are found in step 288 and optimizationends in step 290. Further, a minimum threshold of V_(ddl) fanin conesize k_(min) may be obtained, incrementally, or a total level converternumber upper bound may be incrementally increased to gradually reach anoptimum placement. So, the bound may be incrementally increased,gradually removing least efficient level converters, i.e., by settingk=1 first, then k=2, 3, and so on until k=k_(min) or until a selectedtotal level converter number requirement is met. It should be noted alsothat using V_(ddl) fanin cone size as described herein as a levelconverter efficiency metric is for example only and not intended as alimitation. Any other measurement metric may be employed to iterativelyselect and delete less efficient level converters.

FIGS. 9A-B show before and after examples, 300, 302, respectively, oflevel converter placement optimization effected with logic replacement,i.e., replacing selected V_(ddh) gates with its V_(ddl) counterparts(possibly using a different size in the library) to reduce the number oflevel converters. In particular, this is effective for those V_(ddh)gates that have many fanin signals originating with level converters. Sofor example, in before circuit 300 gate 304 is assigned to V_(ddh),because it is timing critical due to another input from a V_(ddh) gate306. The gate 304 receives its four other inputs from gates 308, 310,312, 314 that are all V_(ddl) cells and so, require insertion of fourlevel converters 316, 318, 320, 322. Thus, in optimized circuit 302,gate 304 is replaced with a functionally equivalent V_(ddl) gate 324and, typically, a level converter (not shown) is inserted at output 326.In addition, the replacement V_(ddl) gate 324 may be of a differentdrive strength. However, the number of level converters may besignificantly reduced by such replacement.

FIG. 10 shows a flow diagram showing an example of the logic replacementstep 330 according to a preferred embodiment of the present invention.First, in step 332 a V_(ddh) gate candidate with multiple input levelconverters is identified. Then, in step 334 the selected V_(ddh) gate istemporarily replaced with its V_(ddl) equivalent. Unnecessary levelconverters are deleted from the inputs to the replaced gate and, ifnecessary, a level converter is inserted at the gate output. Then instep 336, the timing constraint is checked to determine if it is stillmet. Optionally, step 334 may be repeated, trying different V_(ddl) gatesizes and selecting the best result for timing/power. If timing is metin step 336, then the logic replacement with the most power saving isselected in step 338. Otherwise, in step 340 the previous (original)solution is restored. In step 342 the logic is checked to determine ifmore V_(ddh) candidates remain. If so returning to step 322 the nextV_(ddh) candidate is selected, until in step 342 no candidates remainand so, all candidate V_(ddh) gates with multiple level converters inits inputs are checked.

FIGS. 11A-B show before and after examples 350, 352, wherein a buffer354 and level converter 356 are replaced, with a single level converter358 and placement is adjusted to meet design objectives. As notedhereinabove, since each level converter is itself a buffer, levelconverters can be substituted for traditional buffers, e.g., as signalrelays to break long interconnects and restore/redrive signals, therebyreducing buffers or chains of inverters.

FIG. 12 shows a flow diagram 360 for identifying paired level convertersand buffers for optimization. First in step 362, a each level converteris identified with at least one buffer immediately before it with fanout1 (FO1). If such a level converter is identified, then in step 364 thebuffer is temporarily removed, and the level converter placement isadjusted as described hereinabove. Then in step 366, the timingspecification is checked and, if still met, the buffer is permanentlyremoved. Otherwise, in step 368, the original placement is restored.Then, in step 370 the remaining buffers are checked for more candidatesand, if one is found, returning to step 364, that candidate is checked.Otherwise, checking ends in step 372.

A design may be constrained wherein portions may not be modified, e.g.,with input/output (I/O) constraints that may not be replaced, forexample, with V_(ddl) cells. For example in a microprocessor coredesign, placing slower V_(ddl) cells at the input logic between primarychip input and the first level latches, as well as at the output logicbetween the final level latches and the primary chip outputs may beunacceptable. Such constrained logic can be hidden or removed fromconsideration to avoid changing those cells to V_(ddl) cells. Also, auser may specify a supply voltage for a set or sets of cells or macros.Such constraint information can be passed to voltage assignment withthose constrained cells marked as hidden and so, not touched. Also,circuitry related constraints, can be applied during voltage assignment.

Advantageously, the present invention provides a flexible, systematicmethod for identifying cell candidates and creating optimized voltageislands. Further, such a design is achieved with a fine-grained voltageisland and without performance degradation. Additionally, voltageassignment is both logically and physically, honoring both logic andphysical adjacencies. Level converters are efficiently optimized for thedesign.

While the invention has been described in terms of preferredembodiments, those skilled in the art will recognize that the inventioncan be practiced with modification within the spirit and scope of theappended claims.

1. An integrated circuit (IC) comprising: a plurality of circuit rows,at least one row of said plurality of circuit rows including three ormore voltage islands; at least one low voltage island in said at leastone row and at least one low voltage island spans two or more of saidplurality of circuit rows, circuit elements in each said at least onelow voltage island being powered by a low voltage (V_(ddl)) supply; andat least one high voltage island in said at least one row, circuitelements in each said at least one high voltage island being powered bya high voltage (V_(ddh)) supply, V_(ddh) being a higher voltage thanV_(ddl).
 2. An IC as in claim 1 wherein said at least one low voltageisland is a low voltage macro.
 3. An IC as in claim 1 wherein said atleast one low voltage island is a low voltage latch.
 4. An IC as inclaim 1 wherein said at least one low voltage island is a low voltagecell.
 5. An IC as in claim 1 wherein said at least one low voltageisland is surrounded by a plurality of high voltage islands.
 6. An IC asin claim 5 wherein said plurality of high voltage island include a highvoltage standard cell, a high voltage latch and a high voltage macro. 7.An IC as in claim 1 wherein said at least one high voltage islandincludes at least one level converter receiving an output from said atleast one low voltage island.
 8. An IC as in claim 7 wherein said atleast one low voltage island comprises a plurality of low voltageislands and said at least one level converter comprises a plurality oflevel converters in said high voltage island receiving outputs from saidplurality of low voltage islands.